The invention relates generally to methods for monitoring and controlling processes used in forming features on patterned substrates. More specifically, the invention relates to an optical diagnostic method for detecting an endpoint in patterned substrate processing.
In semiconductor manufacturing, various combinations of processes such as etching, thin-film deposition, and chemical-mechanical polishing are used to form features on a patterned substrate. The features are formed by selectively removing materials from and selectively depositing materials on the surface of the patterned substrate. While forming the features, the patterned substrate is monitored to determine when an endpoint has been reached in the process. An endpoint could be a point at which the process conditions should be changed or a point at which the process should be stopped.
The ability to accurately detect an endpoint while processing a patterned substrate is becoming increasingly important as pattern geometries shrink and dimensional control on small feature sizes become increasingly stringent. For etching processes, the ability to accurately detect an endpoint is crucial when the layers of materials to be removed from the patterned substrate are very thin and/or some of the layers on the substrate must remain substantially unaffected after processing of the substrate. For example, in gate etch processes, multiple layers of materials must be removed without damaging the gate oxide layer.
Optical diagnostic methods are typically used to detect endpoints in patterned substrate processing because they are non-intrusive. Optical emission spectroscopy is one example of an optical diagnostic method that detects an endpoint by monitoring emissions from a plasma. The plasma emissions are monitored for the presence or absence of one or more active species. The response of this method is usually delayed because it detects the plasma state instead of the substrate state. Thus, optical emission spectroscopy is generally unsuitable for etching applications where a sacrificial layer that marks an etching endpoint is absent or where an effective etch stop layer is so thin that the chances of etching through it prior to detection of the active species in the plasma is fairly high.
Single-wavelength interferometry is an example of an optical diagnostic method that detects an endpoint by monitoring relative changes in the vertical dimensions of features on the patterned substrate. The method involves directing a narrow light beam onto the substrate surface and measuring the intensity of the beam reflected from the substrate surface. The basic assumption in this method is that the intensity of the reflected beam varies primarily as a result of changes in the feature of interest. By monitoring modulation of the reflected beam, the relative changes in the vertical dimension of the feature of interest can be determined. Because single-wavelength interferometric approaches monitor relative changes in vertical dimensions of features as opposed to absolute vertical dimensions of the features, they are limited in their ability to compensate for incoming material variations, such as variation in thickness of layers formed on substrates, variation in starting depth of trenches, variation in pattern densities, and variation in wafer orientation.
Spectroscopic ellipsometry, polarimetry, and reflectometry are examples of optical diagnostic methods that can be used in conjunction with rigorous optical modeling techniques to determine the absolute vertical and lateral dimensions of features of special test structures such as one-dimensional gratings on a patterned substrate. However, these techniques are limited to in-line metrology applications (i.e., pre- and post-processing metrology) rather than in-situ diagnostics since they involve measurements only on special test structures and also a significant computational load. Efforts have been made to combine the use of spectroscopic ellipsometry and simple, considerably less accurate, modeling techniques for in-situ diagnostics.
From the foregoing, there is desired a robust, easy-to-use, and accurate method for in-situ diagnostics that will facilitate detecting an endpoint in patterned substrate processing without any special test structure requirements.